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Manganese dioxides of different crystalline structures with different dimensionality (amorphous MnO 2 nanoparticles, R-MnO 2 nanoparticles, and β-MnO 2 nanowires) were synthesized, characterized, and evaluated for their electrocatalytic activity to H 2 O 2 . Via a direct and facile electrochemical deposition method, MnO 2 nanomaterials were codeposited onto glassy carbon (GC) electrodes with chitosan hydrogel. The catalytic oxidation current of amorphous MnO 2 nanoparticles per unit mass to H 2 O 2 is much larger than that of R-MnO 2 nanoparticles and β-MnO 2 nanowires; however, the catalytic oxidation current of amorphous MnO 2 per unit surface area to H 2 O 2 is the same as that of R-MnO 2 nanoparticles and much less than that of one-dimensional β-MnO 2 nanowires. The bicatalytic activity toward H 2 O 2 of the electrodes modified with three different nanomaterials increases in the following order: amorphous MnO 2 > R-MnO 2 > β-MnO 2 . Further codeposition of chitosan hydrogel, choline oxidase (ChOx), and different MnO 2 nanomaterials onto GC electrodes was applied to form choline biosensors. The biosensors modified with crystalline MnO 2 respond to choline far more quickly than that modified with amorphous MnO 2 in amperometric measurements. For the same concentration of choline, the response time is 8 s, 25 s, and 5 min for biosensors modified with β-MnO 2 , R-MnO 2 , and amorphous MnO 2 , respectively. The reasons for these phenomena were discussed in detail from the differences of the specific surface areas, the amounts of entrapped MnO 2 on electrodes, crystalline structures, and dimensionality. The biosensors based on R-MnO 2 nanoparticles and β-MnO 2 nanowires were applied on amperometric detections of choline chloride with the linear ranges of 2.0 × 10 -6 -5.8 × 10 -4 M and 1.0 × 10 -6 -7.9 × 10 -4 M with the detection limits of 1.0 and 0.3 µM, respectively.

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We report a novel approach for selective determination of p-phenylenediamine in hair dyes using b-MnO 2 nanowires modified glassy carbon (GC) electrodes through an electrochemical co-deposition process with chitosan hydrogel. A special CE (chemical reaction and electron transfer) process on the surface of b-MnO 2 nanowires modified GC electrode is proposed and proved by cyclic voltammetry and UV-Vis spectroscopy in the presence of pphenylenediamine. p-Phenylenediamine can react with MnO 2 nanowires to produce diimine and the equilibrium of the two-electron and two-proton redox process of p-phenylenediamine on the electrode is changed, and consequently the reductive current is enhanced significantly. At a constant potential of 0 V vs. SCE, other main components of hair dyes including o-, m-phenylenediamine, catechol, resorcinol, and p-dihydroxybenzene do not interfere in the determination of p-phenylenediamine in the amperometric measurement because of their much lower chemical reaction activities with MnO 2 nanowires. It shows a determination range of 0.2 -150 mM and a low detection limit of 50 nM to response p-phenylenediamine. This modified electrode is successfully used to analyze the amount of pphenylenediamine in hair dyes without preseparation procedures.